There are numerous prior art methods for coating substrates to improve their performance, e.g. lifetime, abrasion wear resistance and similar properties. For example, consider the case of plastic sunglass lenses or plastic prescription eyewear. Due to the ease of scratching plastic, abrasion-resistant coatings are desired which can protect the surface of plastic lenses and extend their useful lifetime. The attributes desired for such a coating are high transmission of visible light, high clarity, total absence of color, abrasion protection as good as or better than glass, chemical protection in case of safety glasses, and ability to withstand moisture, heat, and UV radiation.
Non-brittle behavior, associated with flexibility, is also desirable so that degradation in the impact resistance of the lens is avoided, and deep scratches are less noticeable. Other optically transmissive applications may require outright flexibility. Flexibility or conversely brittle behavior, can be quantified by stretching or bending a sample with the coating on the convex surface and measuring the % elongation (100.DELTA.L/L) at which the coating fails, i.e. develops fine cracks. This will be referred to herein as the strain to microcracking. Coatings for aircraft windows, for example, must have at least 1% strain to microcracking. Likewise windows for boat sails or automobile convertible tops should be flexible enough that they can be folded without incurring permanent damage. To make such a coating marketable, the process of deposition must be rapid, inexpensive, reliable and reproducible.
Plastic lenses sold into the ophthalmic lens market are largely coated by acrylic and polysiloxane dip-coatings or spin coatings. These coatings significantly improve the abrasion resistance of the coated lens compared to the uncoated lens. This is particularly true for the case of polycarbonate which is very subject to abrasion. However, improved abrasion resistance of coated lenses is still a major problem in the ophthalmic lens industry. The industrial goal is to obtain plastic lenses which exhibit the same abrasion resistance as glass lenses. Current commercial plastic lenses have abrasion resistance characteristics which are poor compared to glass. Therefore, when purchasing lenses, one must choose between glass, which is very abrasion-resistant but is heavier, or plastic which is lighter but much less abrasion-resistant.
It is well known that plasma deposition produces coatings that can provide much better abrasion resistance, chemical inertness, and the like, than coatings generated by the wet chemical methods mentioned above. Very hard, amorphous coatings can be readily made, for example, by placing the substrate on the capacitively coupled ("powered") electrode in a radio frequency reactor, and exposing the substrate to a high-power and low-pressure methane plasma. Such coatings, commonly referred to as diamond-like carbon (DLC), can likewise be produced by exposing the substrate to a beam of energetic hydrocarbon ions generated at very low pressures in an ion source.
In the radio frequency plasma, as in the ion beam process, the substrate is also irradiated by energetic ions from the plasma, due to the presence of a large negative bias voltage on the powered electrode. When substrates are mounted remotely from the powered electrode, they experience only low energy ion irradiation (less than 20 eV). The properties of the coatings are sensitively dependent on the energy of the ions, i.e., bias voltage, which can be controlled by adjusting radio frequency power and pressure. A low bias voltage will generally produce polymeric coatings which are soft, yellow, and have a strain to microcracking beyond 5%, and have low internal stress. Under high bias voltages, on the other hand, the coatings are very hard (hence the term diamond-like), black, brittle, and have high internal compressive stress.
There are many techniques wherein the deposition takes place without energetic ion bombardment. In these so-called plasma polymerization processes, control of coating properties such as hardness are primarily achieved by selecting the appropriate precursor gas chemistry, substrate temperature, and W/FM parameter. The W/FM parameter is the plasma energy per unit mass of monomer, where W is the power, F is the volume flow rate, and M is the molecular weight of the precursors; see Sharma A, and Yasuda H., J. Appl. Polym. Sci., vol. 38, page 741 (1980). On the other hand, ion-assisted processes have the additional parameter of ion bombardment which provides one with more control of the coating properties and hence a wider choice of process conditions. Ion bombardment affects not only the density and hardness, but also the morphology of the coating which determines the optical clarity, i.e. the degree to which the coating scatters light and appears hazy.
It is commonly known that protection of soft substrates from abrasion caused by fine grit, as is found in the CS10F Taber wheel used in abrasion testing, requires coating thickness in excess of 1 micron (micrometer). Due to the high compressive stress it is difficult to deposit DLC on soft plastics such as polycarbonate to thicknesses greater than 0.5 microns without the formation of stress cracks. DLC is therefore unsuitable as an abrasion protective layer on such substrates. While the softer plasma polymers can be deposited to much greater thicknesses, coatings of this type made from hydrocarbon precursor gases can not be used in applications requiring water-white coatings.
It is well known that the color of the coating is greatly reduced if organosilicon feed gases are used instead of hydrocarbons. However when using alkylsiloxane or alkylsilazane feed gases, which respectively have Si--O--Si or Si--NH--Si linkages, the abrasion resistance is not much better than that of wet chemical polysiloxane coatings. It is also well known, on the other hand, that if these monomers are mixed with oxygen, the coating hardness increases. It is known that the use of alkoxysilanes (which have Si--O--C linkages), also produces harder coatings.
Because it is generally believed that coatings as hard as glass are required in order to achieve glass-like abrasion resistance, the emphasis in the prior art has been to use dilute mixtures of silanes or organosiloxanes in O.sub.2, or to use organosilicon precursors rich in alkoxy substituents. Much of the prior art was done by plasma polymerization techniques without substrate bias. Ion bombardment is beneficial in that it enhances the surface mobility of the depositing species and leads to smoother and less hazy coatings. In plasma polymerization techniques such as microwave deposition, the same effect can be achieved by increasing the substrate temperature, however, this is limited by the temperature stability of the substrate. Furthermore, deposition of hard coatings by plasma polymerization requires high W/FM and low pressures conditions, and thus the deposition rates are typically much lower than in the ion-assisted methods. While microwave deposition can produce rates similar to ion assisted methods, high discharge power density can produce submicron particles in the plasma, which in turn can lead to hazy coatings; see Wrobel, A. M., Kryszewski, M., Progr. Colloid Polym. Sci., vol. 85, page 91 (1991).
The following references illustrate prior art coating processes and abrasion-resistant coatings:
Zehender et al., U.S. Pat. No. 4,085,248, describe a plasma polymerization method for making a protective coating for an optical reflector by first coating the reflector with evaporated A1 and then depositing an organosilicon layer from a hot filament generated electrical discharge.
Kubachi, U.S. Pat. No. 4,096,315, discloses a process for producing a protective coating on an optical polymer substrate that includes plasma polymerization of an organosilicon gas, followed by exposure to a non-depositing plasma for crosslinking and stress relief. Kubachi teaches that the intrinsic stress of his coatings limits their thickness on plastics to less than 3 .mu.m.
Kaganowicz, U.S. Pat. No. 4,168,330, describes a process for depositing a silicon dioxide layer on a substrate by activating a mixture of cyclic siloxanes and oxygen "around the substrate by means of a glow discharge." It is taught that this plasma polymerization process was designed for depositing thin dielectric layers on audio/video discs.
Letter, U.S. Pat. No. 4,217,038, discloses and claims an oxygen permeable, soft, and flexible contact lens comprising a polysiloxane lens and a radio frequency sputter-deposited SiO.sub.2 layer which is thinner than 8000 .ANG.. Letter teaches that this coated lens can be folded or bent back upon itself without breaking. In the deposition process the lens was not biased, and thus the permeable coating was evidently not fully dense. In any event, it is well known that such thin coatings will provide little abrasion resistance.
Tajima et al., U.S. Pat. No. 4,649,071, disclose and claim a substrate with a two component graded single layer or multilayer structure, with more adhesive component on the substrate side of the coating and more protective component on the other side. The change in coating properties is achieved, in this plasma polymerization process, purely by changing feed gas composition.
Kieser et al., U.S. Pat. No. 4,661,409, describe a microwave deposition method for producing hard DLC coatings over large surface areas, using siloxane or silazane interlayers for improved adhesion.
Enke et al., U.S. Pat. No. 4,762,730, disclose and claim a biased radio frequency plasma process for depositing a transparent protective coating on an optical plastic substrate. Made from a mixture of either siloxane or silazane monomers and oxygen, with the oxygen partial pressure at least five times greater than that of the monomer, this process produces essentially silicon dioxide coatings that are as hard or harder than quartz.
Ovshinsky et al., U.S. Pat. No. 4,777,090, describe a microwave deposition process for making a compositionally graded coating on soft substrates consisting essentially of carbon at the substrate and SiO.sub.x remote from the substrate, where x is from 1.6 to 2.0. The role of the carbon interfacial layer is to improve adhesion. Data was presented which indicates that these coatings can be hazy at the level of 5%.
Sliemers et al., U.S. Pat. No. 4,778,721, disclose a method for making an abrasion-resistant plasma coating by restricting the monomer to alkoxy substituted silanes, or mixtures of these with 30% or less of oxygen. It is taught that these coatings are much harder than those obtained from conventional organosilicon monomers, such as hexamethyldisiloxane. Also according to the teachings, these coatings can be made in a variety of "conventional" plasma reactors, but the inventors refer specifically to the reactor of Fletcher et al., U.S. Pat. No. 3,847,652, which has no provisions for biasing the substrate.
Custer et al., U.S. Pat. No. 4,783,374, disclose a coated article with a plasma generated abrasion-resistant and substantially transparent coating made from a precursor gas mixture of a silane, an alkene, and oxygen. Both radio frequency and microwave deposited coatings are discussed. According to the teachings, "reducing the radio frequency power to less than 200 W (0.6 W/in.sup.2 cathode area) reduces yellowness". The "glass-like" abrasion performance demonstrated in this patent was therefore achieved with a yellow coating.
Benz et al., U.S. Pat. No. 4,830,873, disclose and claim a process for making thin transparent coatings by applying a monomeric vapor of organic compositions, and forming a protective layer from an electrical gas discharge by means of polymerization from the vapor phase with the assistance of radiation, followed by the addition of substances which improve the layer hardness. It is taught that the preferred range for organosilicon and oxygen mixtures is 1:8 to 1:16 which produces hard SiO.sub.2 -like material.
Devins et. al., U.S. Pat. No. 4,842,941, disclose and claim a method for making an abrasion-resistant coating on polycarbonate by using a wet-chemical produced interlayer and then plasma depositing an inorganic top layer such as SiO.sub.2.
Reed et al., U.S. Pat. No. 4,927,704, disclose and claim a plasma method of making an abrasion-resistant coating with a graded structure which changes from the substrate outward from organosilicon material to abrasion-resistant inorganic material. The strain to microcracking was measured for two coatings of this invention. Deposited on polycarbonate, and consisting of plasma deposited organosilicon and SiO.sub.2 layers, both coatings failed at strains less than 0.5%.
Brochot et al., U.S. Pat. No. 5,061,567, disclose and claim a glass object with metal or metal-oxide layers upon which a thin organosilicon coating is plasma deposited for corrosion protection.
Bonet et al., U.S. Pat. No. 5,093,152, describe a plasma polymerization method for making a coating of composition SiC.sub.0-5 N.sub.0.3-0.8 O.sub.1.3-2.5 H.sub.0.5-1.2 on plastic optical substrates, by placing the substrate in the afterglow of a plasma and injecting a silicon-containing material near the surface of the substrate.
Kimock, et al., U.S. Pat. Nos. 5,135,808, 5,190,807, 5,268,217 disclose direct ion beam deposition processes using a hydrocarbon gas or carbon vapor for producing abrasion wear resistant products comprising substrates with hard outer coatings of substantially optically transparent diamond-like carbon (DLC) useful for commercial articles such as optical lenses, sunglass lenses, and bar code scanner windows.
Lopata et al., European Pat. No. 0299754, disclose a plasma polymerization method for depositing a "silicon oxide based coating", by exposing the substrate, which is electrically isolated from the system, to a plasma containing an organosilicon compound, oxygen, and an inert gas.
Schmidt and Angus, European Pat. No. 0395198, disclose a composition of matter, which is hydrogenated or nonhydrogenated DLC containing small quantities of silicon, boron, oxygen, or fluorine. The patentees teach in detail how to control hardness, lubricity, density, electrical conductivity, permeability, adhesion and stress, but do not discuss optical properties nor other mechanical properties such as extensibility.
d'Agostino et al., European Pat. No. 0528540, disclose a radio frequency plasma method of making abrasion-resistant coatings from fluorinated cyclic siloxanes.
Relative to optical applications for plastics, the coatings of the prior art listed above suffer from one or more of the following shortcomings:
(1) Not highly transmissive in the visible range. PA1 (2) Not water white. PA1 (3) Not clear (hazy). PA1 (4) Less abrasion-resistant than glass. PA1 (5) Highly stressed (&gt;5.times.10.sup.9 dynes/cm.sup.2) which limits coating thickness and causes bow in the substrate. PA1 (6) Too soft (&lt;2 GPa). PA1 (7) Highly crosslinked, therefore hard but brittle, with hardness equal to or greater than glass (.about.6 GPa), and strain to microcracking of less than 1%. Brittle failure in very coarse abrasion produces wide microcracked areas along scratches and leads to enhanced and objectionable scattering of light. PA1 (8) Not weatherable, in particular not stable to UV exposure. PA1 (1) Hardness, as measured by a Nanoinstrument, Inc. nanoindentor with displacements in the range of about 50 to about 200 nanometers, of about 2 to about 5 GPa; PA1 (2) Strain to microcracking more than 1%; and PA1 (3) Transparency greater than 85% throughout the visible spectrum.